Add Gemfile
Ruby 2 named params
Indicate the public interface of pattern_parser is only #parse
Rename Combiner to PatternExpander.
Refactor to use CombinationIndex.
Purge alternate method of computing combinations.
PatternExpander takes a pattern on initialization.
Renaming CombinationIndex vars and methods to improve clarity.
Rename element_arrays to element_lists
Refactor combination_index_spec for clarity